1. Field of the Invention
The present invention relates to track seeking operations in a disk drive. More particularly, the present invention relates to a method of determining when to inhibit and when to enable write operations during a head settling period following an actuator seek operation in a disk drive.
2. Description of the Prior Art
Disk drive systems typically include: a disk having a thin magnetic coating upon which user data and position information is stored in the form of flux transitions disposed in a series of concentric tracks; a spindle assembly, having a spindle motor and an associated driver circuit, for supporting and rotating the disk in response to a spindle command signal; a read/write head for detecting the flux transitions in the magnetic material as the disk is rotated relative to the head, and for generating an analog read-back signal carrying data and position information in response thereto; a head-arm assembly for supporting and moving the head radially over the surface of the disk; and an actuator assembly, usually comprising a voice coil motor and an associated driver circuit, for driving the head-arm assembly in response to an actuator command signal in order to position the head relative to the tracks of the disk. Modern drive systems also include a servo system for controlling the position of the read/write head relative to the tracks of the disk. Components of the servo system typically include: a position error channel for receiving the read-back signal provided by the head via an arm electronics unit, and operative to demodulate and decode the read-back signal to generate a position error sensing (PES) signal; and a servo controller unit responsive to the PES signal, and operative to provide the actuator command signal and the spindle command signal to the actuator assembly and spindle assembly respectively.
In digital servo control systems, position information is sampled at discrete times. Modern servo controllers, which may be implemented by a microprocessor or a digital signal processor, use state estimators to determine position, velocity, and acceleration parameters of the head as the head is moved radially over the disk by the actuator and head/arm assembly.
A dedicated servo method is commonly used with multiple disk systems in which a servo head of a single dedicated servo disk surface controls movement of corresponding data read/write heads of a multiple platter disk drive. The entire surface of one side of the dedicated servo disk is pre-recorded with servo track information. The position of the servo head relative to the dedicated disk surface is used to indicate the position of the multiple data read/write heads relative to their corresponding disk surfaces. In a sector servo system, the tracks of the disk surface are divided into radial sectors having a short servo track information area followed by a data area. The servo track information area typically includes a sector marker, track identification data, and a servo burst pattern. The sector marker indicates that servo information immediately follows in the track. In both the dedicated servo and sector servo types of systems, a PES signal is used to generate a corrective input signal that is applied to the read/write head positioning servo.
Servo track information usually includes: a synchronization field, such as for automatic gain control (AGC) or similar signal detecting purposes; a track identification field (TID field) typically comprising a digitally encoded gray code; a PES pattern field generally containing a servo pattern burst; and a customer data identification field generally including an identification synchronization pattern, identification data, and customer data.
The PES field provides for generation of the PES signal as the head reads the PES field. The PES signal typically consists of three or four staggered bursts of transitions, (e.g., A, B, C and D bursts). The PES signal, which is proportional to the relative difference of the positions of the center of the servo head and the nearest track center, is a corrective signal providing an indication of which direction the head should be moved to during either track seeking or track following operations. The measured amplitude of the bursts indicates whether or not the head is in position.
A servo system operates in several modes generally including a track following mode and a seek mode. In the track following mode, the servo controller maintains the head in a path over the centerline of a selected track to facilitate accurate reading and recording of data in the track. In the seek mode, the servo controller is directed to place the head on a target track different from the present track. A seek operation includes an acceleration sequence, and a deceleration sequence followed by a head settling period. As further explained below, during the head settling period, the head may overshoot the center of the target track and then oscillate about the center of the track before settling.
The effect of servo surface irregularities or defects in the PES signal and hence on servo system performance can be severe. Large track misregistration contributions, excessive noise leading to unreliable operation, and non-optimal seek performance are the most significant effects. In order to improve linearity, and reduce sensitivity to disk surface effects, most position channels employ a quadrature technique, and some use a servo head twice as wide as the desired data track spacing. The essence of a quadrature system is the two-position error signals, often called normal and quadrature, are demodulated. The signals are derived from two sets of patterns which, when demodulated, produce position error signals that are in space (X direction) quadrature to each other. Having two signals allows use of only the most linear part of each. The normal and quadrature position error signals are quadrature signals because they are cyclic and out of phase by ninety degrees (one quarter phase).
Track misregistration has two related aspects. Write-to-read track misregistration is the misregistration, for whatever cause, between the centerline of a recorded track and that of the read-back head following a seek to a desired record. Write-to-write track misregistration is the misregistration between a recorded track and an adjacent track, resulting in track encroachment or track-to-track squeeze depending on the direction of the misregistration on the adjacent recorded track. Among the physical factors that can result in track misregistration of both types are thermal track shift, incomplete head settling following a track seek, apparent run-out of the head/track combination due to spindle bearing and arm vibration of frequencies outside the capabilities of the servo system, and errors in the servo position detection circuits. The present application is principally concerned with reduction of track misregistration caused by incomplete head settling following a track seek.
FIG. 1 shows a graph at 10 illustrating sampled values 12 of an exemplary position error signal 14 plotted against a time axis 16 during sampling intervals, having a period T, concurrent with a head settling period following a track seek operation of a typical prior art servo system. The sampled values of the PES signal are typically binary values including X bits. The depicted graph also includes: a center value 18 indicative of the center of the target track to which the head is settling; a positive error limit 20; and a negative error limit 22. The positive and negative error limits, forming an error margin, are used in a prior art process of inhibiting write operations during track seeking. In accordance with typical prior art method, write operations are inhibited during track seeking and track settling until a specified number, X, of consecutive sampled PES values 12 of the position error signal 14 are determined to be within the specified error margin 20, 22 from the center value 18 indicating that the read/write head is within a specified distance from the center of the target track. Prior art methods typically require X=4 or 5 consecutive sampled PES values to be within the specified error margins before enabling write operations.
In the depicted example, a first sampled PES value 12, sampled at time t.sub.0, is greater than the positive error limit 20. However, the next fifteen PES values, sampled at times t.sub.1 -t.sub.15, are within the error margin 20, 22. Therefore, assuming that the servo system enables write operations after X=4 consecutive sampled PES values are within the specified error margin, the prior art servo system enables write operations at time t.sub.4 for the depicted PES signal 14. However, in the depicted example, it is actually safe to begin write operations at t.sub.1 because the next 14 sampled PES values, sampled at time t.sub.1 -t.sub.14, are all within the specified error margin. Assuming a sampling period of 100 .mu.sec, the delay incurred by the prior art servo system in enabling write operations would be equal to the time elapsed between time t.sub.1 and time t.sub.4 which is approximately equal to 400 .mu.sec.
An additional problem in prior art disk drives is that during the settling period, the head may settle to positions within the error margin for a period and then abruptly move to a position outside of the error margin. In the depicted example, the PES value 12 sampled at time t.sub.16 is outside of the negative error limit 22. However, in accordance with the prior art method of inhibiting write operations, the write operations enabled at time t.sub.4 are still enabled at time t.sub.16, and therefore a higher TMR occurs if the head writes at time t.sub.16 because the head is beyond the desired limit.
What is needed is a process of enabling write operations during a head settling period following an actuator seek operation in a disk drive, the process providing minimal delay in enabling write operations while protecting against track misregistration due to incomplete head settling.